[Merge] ~ddstreet/software-properties:simple_http_unix_connection into software-properties:ubuntu/master

[Shivaram Lingamneni]

I believe I tested this successfully in 20.04, with minor backporting. The following changes are necessary on top of ecf462b:

1. `sock.sock = sock` should be `self.sock = sock`
2. A reference to `self.LIVEPATCH_RUNNING_FILE` should be to `LIVEPATCH_RUNNING_FILE` instead
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https://code.launchpad.net/~ddstreet/software-properties/+git/software-properties/+merge/396926
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